DESCRIPTION (Applicant's Abstract): The chemistry of NO interactions with hemoglobin has been viewed as a means of mitigating NO vasoactivity (irreversible binding) and eliminating NO from the body (oxidation to nitrate). This presented a conundrum, as by scavenging endothelium-derived NO and thereby decreasing blood flow, Hb seemed to oppose its function in oxygen delivery to tissues. Work from this laboratory, however, has shown that under physiological conditions, Hb does not scavenge NO, but rather introduces new chemistry when pO2 is high that channels NO groups into products, such as S-nitrosothiol (SNO), that preserve its bioactivity. In particular, SNO-hemoglobin (SNO-Hb) has been shown to contribute to the classical systemic responses of hypoxic vasodilation and hyperoxic vasoconstriction. A remaining question is what role NO/Hb chemistry plays in regulation of pulmonary vascular responses. Recent studies on NO/red blood cell (RBC) interactions in the lung have shaped a new perspective on their role in hypoxic pulmonary vasoconstriction (HPV). It appears that RBCs sequester NO augmenting HPV. The challenge is now: to understand the chemistry of NO interaction with RBCs in the lung; to reconcile these data with synthesis of SNO-Hb taking place there; and ultimately to determine if SNO-Hb also regulates HPV. Wepropose that the allosteric environment - pH, pCO2, pO2 (different in pulmonary and systemic arterial blood--dictates whether Hb alternatively functions to store (Fe(III)NO-Hb), consume (Fe(III) - Hb + nitrate), or deliver (SNO-Hb) NO bioactivity. We suggest, moreover, that SNO-Hb opposes HPV. We address this proposal in 4 specific aims, which are: 1) To elucidate the effects of allosteric modulation by pH, pCO2, and pO2 on the interactions of NO with native Hb and RBCs; 2) To determine the ability of allosteric effectors to control the accessibility of SNO and Fe(II)NO complexes in Hb, as measured by functional responses in pulmonary arterial bioassays in vitro; 3) To establish the functional linkage between the allosteric transition, Hb/NO interactions, and pulmonary and systemic hemodynamics in an in vivo model; and 4) To determine SNO-Hb and Fe(II)NO-Hb levels in humans and to establish their relationship to pulmonary vascular tone.